1. Field of the Invention
This invention relates to an electromechanical transducer and, more particularly, to a transducer commonly known as a radial vibrator transducer in which the dominant mechanical motion is in the radial direction of a cylindrical or spherical shaped transducer and which results in an alternate expansion and contraction of the transducer.
2. Description of the Prior Art
A device commonly known as a "radial vibrator" is a simple and widely used electromechanical or electroacoustical transducer type. Such a device in its simplest form consists of a cylindrical or spherical piece of active material which can be driven electrically to induce a radial expansion therein. For example, a tube or ring of a piezoelectric ceramic (such as a lead zirconate titanate formulation) which has electrodes on its inner and outer surfaces and is polarized in the radial direction may act as a radial vibrator. This type of device is usually operated at its first circumferential or "breathing mode" resonance frequency to achieve a higher output.
For a simple cylinder or sphere, the frequency of this resonance is predominantly determined by the type of material and the diameter of the ring or tube. In order to achieve a greater degree of control over the resonance frequency, a number of design schemes are commonly applied which fabricate the ring as a composite structure of alternating segments of active and inactive material. These methods are often implemented by joining bars of the different materials together as barrel staves to form a composite ring. The inactive material generally functions as an added mass and/or an added compliance which acts to lower the radial resonance frequency. An example of a prior art segmented ring radial vibrator is shown in FIG. 1. Piezoelectric material or active staves 1 are bonded to inactive staves 2 forming a composite cylinder and the active staves are electrically wired in parallel so that when a voltage is applied between the electrical leads, the composite cylinder expands or contracts along the radial axis of the device. The arrows on FIG. 1 indicate the direction of polarization and, as illustrated, the electrodes in this structure are located at the boundaries between the active 1 and inactive 2 materials. The device of FIG. 1 may be used as either a generator or receiver of mechanical or acoustic energy and is normally operated in a frequency band approximately centered on its primary mechanical resonance frequency.
It is well known by those of ordinary skill in the art that the performance of the conventional transducer in FIG. 1 can be approximated by the analogous behavior of a simplified electrical equivalent circuit, as shown in FIG. 2. This approximation applies equally as well to a solid ring or a segmented ring as in FIG. 1. In the circuit, M represents the total mass of the ring, and the circumferential compliance of the ring is represented by the capacitor C. C.sub.0 represents the clamped capacitance of the ring and .phi. represents the electromechanical transformation ratio of the active material. The resistor R at the right of the equivalent circuit represents the electric equivalent of the radiation resistance of the medium and the equivalent current u in the resistance R represents the velocity of the moving face of the radiator.
The transmitting voltage response (TVR) of this prior art device is calculated from this equivalent circuit approximation and is proportional to the current u divided by the drive voltage E at the input to the transducer circuit. In determining the response of the device, as expressed by Equation (1) below, the radiator impedance can be neglected. ##EQU1## The transmitting voltage response has a single peak near the frequency where the denominator of the expression becomes zero. This occurs at the resonance (angular) frequency .omega..sub.r as set forth in Equation 2 below: ##EQU2## The method of analysis discussed above is well known in the transducer industry, as discussed in, for example, Leon Camp, Underwater Acoustics, Wiley & Sons, New York, 1970, pp. 136-142; and Butler, "Model for a ring transducer with inactive segments", J. Acoust. Soc. Am., Vol. 59, No. 2, Feb. 1976, pp. 480-482. More complete and accurate performance predictions for transducers can be obtained by using a computer model, such as developed by K. M. Farnham, obtainable from Transducer and Arrays Division, Naval Underwater Systems Center, New London Laboratory, in New London, Conn. A graph of a typical response curve, produced by the above-mentioned program, for the transducer of FIG. 1 is illustrated by curve 20 in FIG. 7.
A significant drawback of the prior art transducer of FIG. 1 is that the resonance frequency and operating bandwidth of the transducer cannot be independently controlled in a given size device. The low mechanical input impedance of this transducer at the radiating face also causes problems when the transducer is used in an array configuration where the input impedance of the radiating face needs to be high. As a practical limit, the mechanical input impedance of the array elements must be maintained higher than the acoustic mutual impedances of the array for all possible operating frequencies, thereby precluding operation in a narrow band near the peak of the transducer response where the mechanical impedance becomes small. The basic device, as shown in FIG. 1, also has significant practical limits on the achievable bandwidth. The operating bandwidth can be changed by decreasing or increasing the thickness of the ring of the active material 1, or by changing the compliance of the inactive staves 2. However, this design technique is limited by the following practical design considerations. As the active material becomes thinner, to increase the operating frequency bandwidth, the device becomes mechanically fragile, a significant drawback in transducers intended for underwater use which must withstand the effects of hydrostatic pressure. Furthermore, if inactive material staves are included to decrease the resonance frequency, the sensitivity and power handling capability of the device will be reduced, which is a significant drawback in applications requiring high acoustic output levels.
In an effort to broaden the operating bandwidth of radial vibrators, a number of additional techniques have been attempted. One technique uses electrical components, such as inductors or capacitors, connected between the electrical terminals of the transducer and the amplifier circuits to tune the response of the device. However, the modification using the special electrical termination can expand the bandwidth to a limited extent at the cost of increased size, weight and complexity. In addition, this method may produce localized high voltages at some circuit nodes requiring costly high voltage isolation and shielding. As with the untuned transducer, the tuned transducer when operated in an array configuration encounters significant practical problems.
Another well known technique for broadening the operating band of a transducer is to use external matching layers. The acoustic impedances of the transducer and the medium are matched through external matching layers as illustrated in FIG. 3. In FIG. 3 the internal active ring 1 is completely surrounded by a matching layer 3 consisting of a liquid which is preferably the same liquid as the medium. The liquid layer is surrounded by a solid ring 4 of a substance such as steel. This method will increase the bandwidth somewhat, as illustrated by curve 21 in FIG. 7, however, the requirement that the layers must conform to the surface and completely cover the device places a significant restriction on the range of operating frequency bands in which this technique can be used. In some applications, the use of a liquid matching layer is undesirable. In these cases, a compliant solid, such as plastic, could be used. However, the shape of the response curve is a fairly sensitive function of the density and speed of sound in the matching layer material making acceptable materials difficult to find. Further, when an external matching layer is used, at least two frequencies occur in the operating band where the head mechanical input impedance becomes unacceptably low for operation in an array configuration. This reduces the usable bandwidth by at least 20 percent.